A couple of times recently I've hit Reply and sent a response to
the originator, rather than the list - for which I apologise. All
other - non-Debian - lists to which I subscribe use this field and
it enables one to concentrate on the message, rather than the
postman:-)

Competent MUAs (like Mutt/elm and Evolution) have a Reply-to-List
command.

I wouldn't call Thunderbird incompetent, but your point is well taken.
Peter HB